chruby-exec: Send the output of chruby in the sub-shell to /dev/null.

This makes test_chruby_exec() pass, and it means you will not see messages like "Using 1.9.3" when you run chruby-exec.
Merge pull request #143 from DavidEGrayson/pull_request_2

chruby-exec: silence messages like "Using ruby-1.9.3"